A slot is a narrow opening in a machine or container, such as a hole for a coin in a vending machine. In computers, a slot is the area where an expansion card can fit into a computer to add more circuitry and capability.

A player inserts cash or, in ticket-in, ticket-out machines, a paper ticket with barcodes, into a slot and then presses a lever or button (either physical or on a touch screen) to activate the reels. The reels then stop to reveal a combination of symbols, and if the player matches a winning line, he or she wins credits based on the amount that he or she had bet before. Modern slot machines are often controlled by microprocessors, which allow manufacturers to assign a different probability to each symbol on each reel.

Although most people who play slots do so as entertainment, some players experience severe gambling problems. These may include financial loss, damage to relationships and careers, and even involvement in criminal activities in support of their gambling behavior. For this reason, it is important to understand what makes slot machines so alluring, so that we can better educate the public about responsible gambling and prevent the occurrence of gambling-related harms.